As the degree of circuit integration has increased, the feature sizes of IC's have dramatically decreased. To support future semiconductor fabrication requirements, lithography systems using charged particle beams, such as electron beams or ion beams, have been developed to overcome feature size limitations of traditional optical systems. In charged particle beam projection lithography systems, portions of a mask are exposed to a charged particle beam to project an image of the mask onto a substrate. Several new charged particle beam lithography systems have been developed to extend lithography capabilities to sub-0.15 micron feature size levels. One such system is a micro column electron beam system developed by IBM. This system uses a large number of miniature electron beam writers in a phased array to project mask images on the order of 0.1 micron wafer geometries. A similar electron beam projection lithography system is known as PREVAIL® also developed by IBM.
Another electron beam lithography system is a SCALPEL® developed by AT&T Bell Laboratories; SCALPEL, stands for “Scattering with Angular Limitation in Projection Electron-beam Lithography” and is a registered trademark of AT&T Bell Laboratories of Murray Hill, N.J. The SCALPEL lithography system exposes a photomask to high-energy electrons to project an image of the mask onto a substrate, coated with an energy sensitive material or resist. During semiconductor fabrication multiple layers are deposited on the substrate and several layers may be exposed to the patterned high-energy electrons.
Proper alignment of the mask image with preexisting features on the wafer during lithography processing is critical because many patterened layers must be aligned within specific tolerances to produce functional integrated circuits. The alignment tolerance or alignment budget of a projected pattern is proportional to the critical dimension (CD) of the circuit. A typically alignment tolerance or alignment budget is CD/3. During processing, the wafer is held on a chuck within the processing chamber typically by vacuum or electrostatic force. To ensure accurate positional registration, points on the wafer are interrogated or measured by the lithography tool alignment systems to automatically determine the locations of the preexisting features. This enables the next pattern level to be accurately positioned.
In conventional optical 248 nm, 193 nm wavelength UV, deep-UV, extreme-UV and electron beam lithography, energy density typically between about 10-25 mJ/cm2 is applied to the wafer. If the energy applied to the wafer during conventional lithography processing is low and the wafer temperature rise is small (less than 0.1° C.), thermal expansion is small compared to the tolerances required at the feature sizes that can be printed with such tools. In lithography systems where the wafer temperature rise is less then 0.1° C., alignment correction due to thermal expansion is typically not required.
In wafer processing systems were thermal expansion needs to be controlled, thermalization of the wafer is a commonly used practice. Thermalization maintains a wafer at a constant temperature by passing constant temperature air over the surface during processing. Because thermalization requires the circulation of air, it is not compatible with vacuum chamber systems.
Some semiconductor and lithography processes, including SCALPEL, expose a smaller area of the wafers to substantially more energy which results in thermal distortion of the wafer. The exposure of the wafer to a high energy particle beam having an energy density of more than 1.0 Joule/cm2 creates local heating at the area of incidence and can increase the local temperature from approximately 1° to 50° centigrade. As the particle beam traverses the wafer and energy is absorbed during lithography processing hot spots on the wafer are produced. These hot spots result in localized thermal expansion of the wafer. In lithography systems which project images with critical dimensions less than 0.15 μm, wafer temperature variations as low as 0.1° to 1° C. can produce enough thermal deformation to cause misalignment.
In lithography systems that expose the wafer to higher energy levels, the wafer temperature may be stabilized by control mechanisms which cool the wafer and reduce thermal expansion. Some wafer chucks have been designed to prevent wafer heating by circulating a fluid under the wafer to cool the wafer during processing. Wafer cooling chucks have been used primarily in systems which expose the entire wafer to plasma or ion beams during processing. Multiple zone wafer cooling chucks have also been developed which monitor the temperature of various areas of the wafer and independently adjust the cooling of each area to maintain the desired uniform water temperature. In general, however, multiple-zone wafer cooling chucks cannot prevent thermal deformation of wafers in high energy particle beam lithography systems because the energy absorbed by the wafer can not be removed quickly enough by conventional convection heat transfer mechanisms to prevent local heating of the wafer and thermal expansion. Also, because the high energy particle beam is quickly scanned across the substrate, the independent cooling zones of the present wafer cooling chucks may not be able to regulate a uniform temperature across the entire wafer to prevent thermal deformation.
The magnitude of thermal deformation of the substrate during lithography is proportional to the change in temperature of the substrate. Because particle beam lithography systems may only heat a small area and scan the wafer, the thermal deformation of the wafer varies throughout the lithography process and is not uniform. Thermal deformation due to heating during lithography processing can result in 10 to 100 nm of wafer movement.
As discussed, during lithography processing the substrate is generally held in contact with a chuck by a vacuum or electrostatic force. Because the wafer is held on a chuck by force, significant friction force can oppose any relative motion between the wafer and chuck including thermal deformation of the wafer. The friction force is dependent on the substrate material, the chuck material, the clamping force and the condition or roughness of the surfaces in contact.
During lithographic processing, the substrate is exposed to a high energy particle beam which produces a substantial amount of heat at the point of incidence. The thermal expansion force of the heated substrate is opposed by the friction forces between the substrate and chuck which prevents the wafer from expanding until the thermal expansion force exceeds the friction force. When the thermal expansion force exceeds the friction force the substrate quickly expands and is prevented from expanding again until the thermal expansion force again exceeds the friction force this type of affect is sometimes referred to as “stick-slip” motion. The relationship of the thermal expansion force and friction force causes the substrate to thermally deform in an incremental manner.
Similarly, as the substrate cools the thermal contraction force builds until the deformation force exceeds the friction force. Again, when the contraction force exceeds the friction force the substrate moves quickly and is prevented from contracting again until the thermal contraction force exceeds the friction force. Like the thermal expansion, the substrate contracts in an incremental manner. Stick-slip is a known problem in the fields of high energy lithography systems, monochromators and frequency stabilized lasers.
Stick-slip causes the thermal expansion of the substrate to be unpredictable and makes alignment of the substrate during processing extremely difficult. Alignment computer systems may be used to compensate for the thermal deformation of a substrate, however current alignment computer systems can not accurately determine the substrate position during thermal deformation having stick-slip motion.